I am a casual observer of the tour so maybe I am missing something. How have the top three riders been seperated by the same interval for the last five stages or so? I cannot see how this is possible unless it is a set figure. Sorry I am just a naturally curious person by nature drives (bikes) by wife crazy.

A pack of riders is given the same time as those around them. It generally takes a gap of 3-5 seconds off the back wheel for them to be given a differnt time. This is done for safety as it not a good idea to have 200 riders clawing for seconds on a 15 meter wide road at the finish.